WASHINGTON, Jan 9 (Reuters) - The White House on Thursday urged both sides in the deadly conflict in South Sudan to sign an agreement to cease hostilities immediately.

South Sudanese rebels rejected a government plan on Wednesday to end a dispute over detainees and unblock peace talks aimed at halting fighting that has killed at least 1,000 people in the world's youngest state.

Three weeks of fighting, often along ethnic faultlines, has pitted President Salva Kiir's SPLA government forces against rebels loyal to former vice president Riek Machar and brought the oil-exporting nation close to civil war.

President Barack Obama's national security adviser Susan Rice said in a statement it was the responsibility of both leaders to make sure their country was not marred by more violence.

"Mr. Machar, in particular, must commit to a cessation of hostilities without precondition," Rice said.

"His continued insistence on the release of detainees as a pre-condition for a cessation of hostilities is unacceptable and runs counter to the express will of the detainees."

Both sides met face-to-face for the first time on Tuesday in Addis Ababa in a bid to agree a ceasefire but faced new delays after Kiir refused a rebel demand to release 11 detainees, who were arrested last year over an alleged coup plot.

On Wednesday, the government proposed to shift the peace talks to the United Nations compound in Juba, enabling the 11 detainees to attend the negotiations during the day and return to custody in the evening.

South Sudan's presidential spokesman said the rebels appeared to reject that.

"The United States is disappointed that the detainees being held by the government of South Sudan have not yet been released," Rice said. "The United States reiterates its call upon President Salva Kiir to release the detainees immediately to the custody of IGAD so that they can participate in the political negotiations." (Reporting by Jeff Mason; Editing by Lisa Shumaker)